The present invention relates to television routers, and more particularly to an input expansion for a crosspoint switch module that enables the crosspoint switch module to accept a large number of inputs that can be routed to one or more of several outputs.
Traditional means of combining differential ECL outputs involve the use of an n.times.1 switch where n is the number of ECL outputs to be combined. The number of active differential ECL outputs becomes n+1, i.e., one for every ECL output to be combined plus the output of the n.times.1 switch itself. In television routing applications a crosspoint switch module is used for switching various video input signals. To increase the density of the crosspoint switch module to accept a large number of inputs, multiple crosspoint switch integrated circuits (ICs) need to be provided within the crosspoint switch module with the ECL outputs being combined to provide the module outputs. Furthermore, it is desirable to shut off current in the ECL output drivers of the inactive crosspoint switch ICs in order to minimize system power consumption.
A presently existing circuit for driving a transmission line is described in U.S. Pat. No. 5,331,206 issued Jul. 19, 1994 to John E. Liron. The Liron circuit has multiple single input crosspoint switch modules coupled to a single output ECL bus for each output of the router. Only one of the crosspoint switch modules provides current to the ECL bus at a time. There are in a large television router up to hundreds of inputs and outputs, but each output needs to be isolated from all of the unselected inputs. The router is made up of many crosspoint switch modules, the outputs of which are combined to form the router output(s), as shown in FIG. 2 of the Liron patent.
What is desired is an input expansion for a crosspoint switch module that combines multiple crosspoint switch integrated circuits (ICs) within the module so that only an active crosspoint switch IC supplies current at the output of the module.